Rheumatoid arthritis (RA) is a destructive inflammatory joint disease associated with increased morbidity and mortality. While biologic therapies have improved treatment responses, many unmet clinical needs remain due to the refractory nature of RA, the high cost of these drugs, and the tendency of the disease to flare despite aggressive treatment. During the previous funding period, we discovered that prior to arthritic knee flare in TNF-Tg mice, the adjacent popliteal lymph node (PLN) expands due to the accumulation of lymph fluid and unactivated-polyclonal CD23+/CD21hi/CD1d hi B cells in inflamed nodes (Bin). New lymphatic vessels (lymphangiogenesis) are then formed that protect the adjacent knee from joint from flare until there is a sudden loss of the lymphatic pulse along the ipsilateral axis extending to the iliac LN, resulting in PLN collapse and Bin translocation fro the follicles to the sinuses, which results in clogging of the lymphatics. Interestingly, knee synovitis that occurs during PLN collapse can be ameliorated by B cell depletion with anti-CD20 therapy, which significantly correlates with increased extrinsic lymphatic drainage without recovery of the lymphatic pulse, which is regulated by eNOS. Most recently, we observed that age-dependent PLN collapse is associated with lymphatic endothelium (LE) and smooth muscle (SM) damage from chronic inflammation driven by iNOS producing macrophages that attach to the luminal surface. Serendipitously, we also found that similar LE-SM damage is induced by contrast-enhanced ultrasound (CE-US) in young TNF-Tg mice, which induces PLN collapse never seen in animals <4-months of age. Thus, we hypothesize that: 1) RA flare mediated by loss of efferent lymphatic flow is due to unrepaired LE-SM damage that compromises the lymphatic vessel barrier allowing squelching of the eNOS driven lymphatic pulse by NO- from iNOS expressing macrophages in the lumen. 2) Selective iNOS inhibition ameliorates arthritic flare secondary to PLN collapse by restoring the lymphatic pulse. 3) Alternatively, arthritic flare is treatable with vasodilator therapy that opens parallel lymphatic vessels to enhance and/or restore lymphatic flow. And, 4) Increased Bin accumulation in PLN, and loss of the lymphatic pulse, occur in human RA flare. To test these hypotheses we propose three Specific Aims that will: 1) establish the role of LE-SM damage/repair and NO- squelching of the lymphatic pulse in PLN collapse and knee flare in TNF-Tg mice; 2) demonstrate the efficacy of selective iNOS and PDE5 inhibitors on arthritic flare in TNF-Tg mice; and 3) provide clinical evidence that Bin accumulation and loss of the efferent lymphatic pulse occurs in human RA flare. As this research program has and will continue to provided paradigm changing information on the mechanisms of RA flare and novel therapies for this condition, the potential impact could be huge in terms of new interventions for refractory disease.